Activated carbon is a commonly used form of carbon and has traditionally been produced from fossil fuel. More recent developments have examined processes for producing activated carbon from renewable resources, such as biomass.
Activated carbon can be produced, in principle, from virtually any material containing carbon. Carbonaceous materials commonly include fossil resources such as natural gas, petroleum, coal, and lignite; and renewable resources such as lignocellulosic biomass and various carbon-rich waste materials. In some embodiments, a renewable biomass is used (at least in part) to produce activated carbons because of the rising economic, environmental, and social costs associated with fossil resources.
Biomass is a term used to describe any biologically produced matter, or biogenic matter. The chemical energy contained in biomass is derived from solar energy using the natural process of photosynthesis. This is the process by which plants take in carbon dioxide and water from their surroundings and, using energy from sunlight, convert them into sugars, starches, cellulose, hemicellulose, and lignin. Of all the renewable energy sources, biomass is unique in that it is, effectively, stored solar energy. Furthermore, biomass is the only renewable source of carbon.
Converting biomass to biogenic activated carbon, however, poses both technical as well as economic challenges arising from feedstock variations, operational difficulties, and capital intensity. There exist a variety of conversion technologies to turn biomass feedstocks into high-carbon materials. Most of the known conversion technologies utilize some form of pyrolysis.
Pyrolysis is a process for thermal conversion of solid materials in the complete absence of oxidizing agent (air or oxygen), or with such limited supply that oxidation does not occur to any appreciable extent. Depending on process conditions and additives, biomass pyrolysis can be adjusted to produce widely varying amounts of gas, liquid, and solid. Lower process temperatures and longer vapor residence times generally favor the production of solids. High temperatures and longer residence times generally increase the biomass conversion to syngas, while moderate temperatures and short vapor residence times are generally optimum for producing liquids. Recently, there has been much attention devoted to pyrolysis and related processes for converting biomass into high-quality syngas and/or to liquids as precursors to liquid fuels.
On the other hand, there has been less focus on improving processes specifically for optimizing yield and quality of the solids as activated carbon. Historically, slow pyrolysis of wood has been performed in large piles, in a simple batch process, with no emissions control. Traditional charcoal-making technologies are energy-inefficient as well as highly polluting. Clearly, there are economic and practical challenges for continuous commercial-scale production of activated carbon, while managing the energy balance and controlling emissions. It would be beneficial if activated carbon production could be efficiently integrated, at small scale, at various biorefinery host plants.
A well-engineered carbon production facility has the potential to create energy beyond that required for production of carbon. Co-locating a carbon production facility at a host facility that can both provide feedstocks for carbon production and use biogas or heat produced from carbon production has the potential to improve environmental impacts and costs for production of carbon and at a host facility where the carbon plant may be co-located.